1. Field of the Invention
The present invention relates generally to an underwater tow cable and, more particularly, to an underwater tow cable having a fairing thereon designed to suppress strumming vibrations capable of being induced in the cable when it is towed at shallow angle with respect to the direction of tow.
2. Description of the Prior Art
In a towed array system transverse vibrations of large amplitude occur due to vortex shedding of the tow cable. These vibrations lead to serious mechanical problems. In general, a typical acoustic oceanographic instrumentation system might include a weighted cable towed from a vessel, and an acoustical array containing one or more hydrophones connected to the tow cable. The tow cable is affected by relative water motions which act on it as it is pulled through the water by the vessel. These relative water motions acting on the tow cable set up a mechanical wave motion in the cable known in the art as "strum". Strum is transverse vibrations similar in nature to the well understood resonant tensioned strings as used on musical instruments.
Strum, or vortex-induced cable vibration, can be induced in a cable which tows at an angle with respect to water flow. Strum occurs as water passes about the tow cable and forms a succession of fluid eddies or vortexes on the lee side of the cable. Each vortex gives rise to a pressure differential on the side of the cable causing a transverse mechanical displacement of the cable. This, in turn, enhances the formation of the next vortex. The organized or coherent action of vortex shedding along a sufficient length of cable is believed to be a primary requirement for the occurrence of cable strum.
In the past, various methods have been employed to suppress the occurrence of strum in tow cables. These various methods have all centered around altering the exterior of the tow cable. However, the alterations tried to date have not been very satisfactory in suppressing strum for many applications of the towed systems. Specifically, the alterations are often not effective at the low incidence angles characteristic of many critical angle towed array systems, or they are not compatible with the handling units of the towed systems.
Consequently, there is a need for an improved design tow cable operable to suppress the occurrence of strum in a towed array system at low incidence angles. In addition, there is a need for an improved design tow cable capable of being easily manipulated with existing handling systems.